Radiotherapy consists of directing beams of ionizing radiation toward a tumor. The radiation (usually high-energy x-rays) is absorbed by the tumor cells causing cell damage. Obviously, the radiation is also capable of damaging the healthy tissue around the tumor, so various techniques are used in order to limit this. The dose is usually delivered in time-separated fractions (i.e. individual treatment sessions), allowing the healthy tissue to recover between fractions, and the doses are carefully collimated so that the amount of healthy tissue that is irradiated is minimized.
Within a single fraction, the dose may be delivered from a number of different directions so that the dose to each region of healthy tissue is reduced. This can be delivered in discrete stages, with the radiation source being repositioned after each stage, or the source can be moved continuously around the patient during delivery of the dose.
Where a beam of radiation is being collimated to a specific area, that area will of course be one that is defined relative to the anatomy of the patient. Therefore, the location of the patient needs to be established in advance of treatment so that the beam can be appropriately directed. Usually, a CT scan of the patient (by which we mean to include both a fan- or pencil-beam slice-by-slice CT scan or a CBCT scan, or “cone-beam CT” scan) will be taken immediately prior to each fraction of a fractionated treatment, while the patient is on the couch ready for treatment. This will identify the patient's exact location with accuracy. Modern radiotherapy machines include cone-beam CT scanning capability to permit this.